Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.8 Electronic industry 897
4) special cleaning processes.
In an aqueous cleaning process, 5
cleaning is achieved by using water or a
water-based cleaning agent. In order to
avoid drying spots, rinsing is carried out
with deionized water. By soldering with
water-soluble solder pastes only hot water
is necessary for cleaning. However, the use
of an alkaline water-based cleaning agent is
effective to remove more critical flux residues
with low or high pressure spray in
cleaning equipment.
In semi-aqueous processes, 4 cleaning is carried out using a high boiling mixture of organic
solvents. Rinsing is achieved by using deionized water. The organic solvents used for
these cleaners are predominantly based on alkoxypropanols, terpenes, high-boiling alcohols,
or mixtures of different chemistries.
In applications based on solvents such as isopropanol or alkoxypropanols 6 a rinse with
deionized water is not imperative, since these chemistries do not leave any residues if used
for rinsing. Especially when using an uniformly boiling solvent-based cleaning agent, vapor
rinsing with solvent vapor is possible. All there mentioned cleaning process will be discussed
more detailed in the following sections.
Less widespread applications in the electronic industry include:
• Cleaning using supercritical carbon dioxide (SCF). 7
• Cleaning using plasma. 7
Figure 14.8.3. PCB cleaning.
SCF cleaning alternative is especially useful for extremely sensitive and complex assemblies.
At temperatures above 31°C and pressures above 73 bar, carbon dioxide transforms
into a supercritical phase. Supercritical carbon dioxide reveals an extremely low
surface tension. Consequently, the wetting of small gaps and complex assembly architectures
can be achieved. However, the contamination that should be removed has to be
nonpolar, and the compounds should be resistant to high pressures. Due to these basic limitations
and the high costs of this cleaning process, this process does not play an important
part in the electronic industry.
The advantage of plasma cleaning is in almost waste-free removal of contaminations.
The contaminations are transformed into carbon dioxide and water. However, this cleaning
process is only suitable for removal of the small amounts of residue from the substrate surface.
Combinations with aqueous cleaning processes are feasible and common in the field.
14.8.2.1 Water based systems; advantages and disadvantages
14.8.2.1.1 Cleaning with DI - water
Cleaning without using any kind of cleaning agent in the electronic assembly process is possible
when water-based fluxes are used for the soldering process. Since these aqueous
fluxes are highly activated, aggressive residues occur after the soldering process. These residues
have to be removed immediately to avoid severe corrosion of solder joints. The use of
water-based fluxes is very common in the US electronic market due to the main advantage
of a large soldering process window for soldering. DI-water is used for this purpose since it
is able to solubilize the polar residues. However, physical energy, such as high-spray-pres-